Janus, Rhea, and Rings

This snapshot was taken by the Cassini spacecraft. The small moon that seems to be hovering over Saturn’s rings is Janus. It’s only about 180 km across. The larger moon is Rhea, which is around 1500 km across. Saturn’s thin outer F ring is visible in front of Rhea, and the top of the moon is visible between the larger A and B rings.

Image Credit: NASA

Saturn and Some of Its Moons

The animation shows the orbits of Saturn’s visible moons Tethys, Janus, Mimas, Enceladus, and Rhea over the observing run in June, 2019 (with elapsed time bar).

Video Credits: NASA / ESA / A. Simon (Goddard Space Flight Center) / M.H. Wong (University of California, Berkeley) / J. DePasquale (STScI)

Saturn, Rings, and Moons

Cassini snapped this picture with its narrow-angle camera. It shows Saturn and its rings seen here nearly edge on. The image also shows the moons Mimas (above the rings), tiny Janus (apparently almost in the rings), and Tethys (below the rings). “Above” and “below” the rings is a matter of perspective. All three moons and the rings orbit Saturn in roughly the same plane.

Image Credit: NASA

Saturn’s Moon Janus

This raw, unprocessed image of Janus was taken on 27 March, 2012, by the Cassini spacecraft using the RED and CL2 filters. The image has not been validated or calibrated.

Janus is oddly shaped with dimensions of roughly 203×185×153 km and is extensively cratered with several craters larger than 30 km. Given its very low density and relatively high albedo, it is probable that Janus is a very porous, icy pile of rocks.

Janus occupies practically the same orbit as the moon Epimetheus. This initially confused astronomers who assumed that there was only one body in the orbit. It was eventually realized that they were trying to reconcile observations of two objects instead of a single, strangely behaving moon.

Image Credit: NASA


A quintet of Saturn’s moons appear in this image taken by the Cassini spacecraft.

Janus (179 kilometers, or 111 miles across) is on the far left. Pandora (81 kilometers, or 50 miles across) orbits between the A ring and the thin F ring near the middle of the image. Brightly reflective Enceladus (504 kilometers, or 313 miles across) appears above the center of the image. Part of Saturn’s second largest moon Rhea (1,528 kilometers, or 949 miles across) is visible at the right edge of the image. The smaller moon Mimas (396 kilometers, or 246 miles across) can be seen beyond Rhea also on the right side of the image.

This view looks toward the sunlit side of the rings from just above the ringplane. Rhea is closest moon to Cassini here. The rings are beyond Rhea and Mimas. Enceladus is beyond the rings.

The image was taken by Cassini spacecraft narrow-angle camera on 29 July, 2011. The view was acquired at a distance of approximately 1.1 million kilometers (684,000 miles) from Rhea and 1.8 million kilometers (1.1 million miles) from Enceladus.

Image Credit: NASA/JPL